Upgrading manual gearbox
08-19-2005, 05:42 AM
#1
Rennlist Member
Thread Starter
Upgrading manual gearbox
I am new to this forum and pretty new to Porsches as well. Within a year I have become the proud owner of a 1986 911 Cabriolet and a 1997 993 C4S (pictures attached). So far the "return on investment" has been great driving pleasure, fast developing friendship with other aficionados and owner's pride of course but I must admit it has also brought a few headaches and for sure a burning hole in my wallet.
My query today is about upgrading the gearbox for my 911.
I have the older balk-ring model and following some shifting difficulties, I had the gear box opened and was told that there are several gears to be replaced (not a surprise considering that the car has had several owners and not all of them may have had the best driving abilities...). Now instead of just replacing what needs to be replaced (assuming availability of parts) I am thinking of upgrading to a 5-gear G50 box instead. I'd like to know how readily available these boxes are (e.g. from crashed cars) and what would be a fair price for one in good condition?
Grateful if you have tips and recommendations as well.
My query today is about upgrading the gearbox for my 911.
I have the older balk-ring model and following some shifting difficulties, I had the gear box opened and was told that there are several gears to be replaced (not a surprise considering that the car has had several owners and not all of them may have had the best driving abilities...). Now instead of just replacing what needs to be replaced (assuming availability of parts) I am thinking of upgrading to a 5-gear G50 box instead. I'd like to know how readily available these boxes are (e.g. from crashed cars) and what would be a fair price for one in good condition?
Grateful if you have tips and recommendations as well.
Last edited by NeufCentOnze; 08-19-2005 at 06:01 AM.
08-19-2005, 10:32 AM
#2
Instructor
G50 gearboxes are not difficult to find used. A decent example will usually cost $1500-$2000 USD.
But it is not a simple exchange of gearboxes for your '86 Carrera. The chassis on the '86 is not the same as that of the 87-89 cars originally equipped with G50 gearboxes.
You would need to modify the chassis and rear suspension torsion bar tube to accept the G50. This is because the dimensions of the two gearboxes are not the same. Plus you would need the rear suspension spring plates, rear torsion bars, shifter assembly, shift rod & coupler, pedal cluster (throttle, brake, and clutch are all part of a unitized assembly), hydraulic clutch operation components, speedometer sending unit, clutch fluid reservoir (shared with brake system), a replacement clutch & flywheel assembly because the G50 uses a larger 240mm diameter arrangement.
It's a very involved procedure to convert your car to accept a G50 gearbox. A rebuild of the existing 915 gearbox in your '86 is much more cost effective and less of a hassle. Properly set up and adjusted, the 915 is a great transmission. I think the 915 suffers from age firstly because of abuse. Secondly, periodic adjustments are needed and sometimes neglected. Adjustment of the shifter and the clutch cable are very important. A friend recently rebuilt his 915 and had some difficulties shifting even after the rebuild. After adjusting the shifter alignment, he said "now it's like a completely different gearbox."
But it is not a simple exchange of gearboxes for your '86 Carrera. The chassis on the '86 is not the same as that of the 87-89 cars originally equipped with G50 gearboxes.
You would need to modify the chassis and rear suspension torsion bar tube to accept the G50. This is because the dimensions of the two gearboxes are not the same. Plus you would need the rear suspension spring plates, rear torsion bars, shifter assembly, shift rod & coupler, pedal cluster (throttle, brake, and clutch are all part of a unitized assembly), hydraulic clutch operation components, speedometer sending unit, clutch fluid reservoir (shared with brake system), a replacement clutch & flywheel assembly because the G50 uses a larger 240mm diameter arrangement.
It's a very involved procedure to convert your car to accept a G50 gearbox. A rebuild of the existing 915 gearbox in your '86 is much more cost effective and less of a hassle. Properly set up and adjusted, the 915 is a great transmission. I think the 915 suffers from age firstly because of abuse. Secondly, periodic adjustments are needed and sometimes neglected. Adjustment of the shifter and the clutch cable are very important. A friend recently rebuilt his 915 and had some difficulties shifting even after the rebuild. After adjusting the shifter alignment, he said "now it's like a completely different gearbox."
08-19-2005, 11:22 AM
#3
Ditto... have a good Porsche specialist rebuild the 915 and it will work fine. The key to the 915 is learning how to shift it properly. Some tips FWIW.
First, don;t grap the **** with a clenched fist like you're yanking the handle on a barcolounger... shift with an open palm... this allows you to direct the shift with the accurate, sensitive muscles in your wrist and hand rather than the large clumsey muscles in your upper arm.
Second, don;t force the shifter into gear... guide it with pressure and let the synchros do the work... this will produce quicker, smoother shifts and prolong life of the box. Imagine the shift gate divided into four quadrents... (3rd gear is upper right, for example) and then apply pressure to the **** at the opposite side... (to shift to 3rd, put pressure on the **** at 7-8 o'clock)
Third, use a separate hand position for the shift into each different gear, and alway use that same motion...this way your train your arm to shift only into the gear you want... for example, when I do the 3-4 shift, I reverse my hand so that by palm is facing away from my body... contacting the **** at the 10 o'clock position and pull back and over... this makes it ergonomically impossible to ever shift into 2nd gear... your arm just doesn't move that way... (catching 2nd gear on the 5th-4th downshift is a common, and very very expensive mistake...)
Fourth, learn to heel and toe. This is challenging but once you get the hang of it, it greatly improves the shifting and fun of driving the car as well as protecting your clutch and gears. If you REALLY want to go the nine yards, learn to double clutch heel and toe... this will take alot of load off the synchros and really improve gearbox life, especially if you are doing more sporting driving. You will need to build up the throttle pedal to H&T properly. You can buy an aluminum heel and toe pedal, screw another pedal on top of the origional one, or simply tape a block of wood to the throttle with black racer's tape... (cheap, easily removeable, and allow you to exactly place the pedal where it is most comfortable to you).
Sorry for the lecture, but if you do this, the 915 will shift as well as any car around and be a hoot to drive.
First, don;t grap the **** with a clenched fist like you're yanking the handle on a barcolounger... shift with an open palm... this allows you to direct the shift with the accurate, sensitive muscles in your wrist and hand rather than the large clumsey muscles in your upper arm.
Second, don;t force the shifter into gear... guide it with pressure and let the synchros do the work... this will produce quicker, smoother shifts and prolong life of the box. Imagine the shift gate divided into four quadrents... (3rd gear is upper right, for example) and then apply pressure to the **** at the opposite side... (to shift to 3rd, put pressure on the **** at 7-8 o'clock)
Third, use a separate hand position for the shift into each different gear, and alway use that same motion...this way your train your arm to shift only into the gear you want... for example, when I do the 3-4 shift, I reverse my hand so that by palm is facing away from my body... contacting the **** at the 10 o'clock position and pull back and over... this makes it ergonomically impossible to ever shift into 2nd gear... your arm just doesn't move that way... (catching 2nd gear on the 5th-4th downshift is a common, and very very expensive mistake...)
Fourth, learn to heel and toe. This is challenging but once you get the hang of it, it greatly improves the shifting and fun of driving the car as well as protecting your clutch and gears. If you REALLY want to go the nine yards, learn to double clutch heel and toe... this will take alot of load off the synchros and really improve gearbox life, especially if you are doing more sporting driving. You will need to build up the throttle pedal to H&T properly. You can buy an aluminum heel and toe pedal, screw another pedal on top of the origional one, or simply tape a block of wood to the throttle with black racer's tape... (cheap, easily removeable, and allow you to exactly place the pedal where it is most comfortable to you).
Sorry for the lecture, but if you do this, the 915 will shift as well as any car around and be a hoot to drive.
08-19-2005, 04:07 PM
#4
Rennlist Member
Thread Starter
Thanks to KLehmann and JCP911S for the informative answers.
I think I will follow your advice and just repair the 915 box. Actually your points make sense: when I bought the car, the 3rd gear was a bit sensitive, but it was fine as long as one would engage carefully from 2nd. Trouble started when I asked for a new shifter guide thinking that it would improve things. It only made it much worse: after that it became even more difficult to go from 2nd to 3rd without any grind, going from 4th to 3rd became an even worse problem (where it was not before) and I could hardly go into 5th anymore (would end up in 3rd instead). The local Porsche dealer/garage tried some adjustments but failed completely and the last resort became to open the box and indeed find some wear but I think if I had not tinkered at all it may have remained OK for a long time. Well now is my chance to reapir the box and make it good for some years. I do hope they will do all the required adjustments right this time!
Regards
I think I will follow your advice and just repair the 915 box. Actually your points make sense: when I bought the car, the 3rd gear was a bit sensitive, but it was fine as long as one would engage carefully from 2nd. Trouble started when I asked for a new shifter guide thinking that it would improve things. It only made it much worse: after that it became even more difficult to go from 2nd to 3rd without any grind, going from 4th to 3rd became an even worse problem (where it was not before) and I could hardly go into 5th anymore (would end up in 3rd instead). The local Porsche dealer/garage tried some adjustments but failed completely and the last resort became to open the box and indeed find some wear but I think if I had not tinkered at all it may have remained OK for a long time. Well now is my chance to reapir the box and make it good for some years. I do hope they will do all the required adjustments right this time!
Regards
08-19-2005, 04:42 PM
#5
Instructor
looking at the picture of your '86 and the fact that you're in Oman i'd say there's an xlnt chance that you have the most desireable model 915...with a trans cooler. keep it, fix it and enjoy.
08-20-2005, 10:17 AM
#6
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Remember, with a 915, it is rarely the synchros that cause grinding; the operating sleeves wear out first. Also, when mechs say 'gears' they usually mean the dog teeth, which are replaceable separatly from the atcual gears. The gear teeth are constant mesh, it is the dogs that wear when grinding occurs.
I agree that a well done 915 is a great robust trans.
It is also a pleasure to work on. Elegantly engineered and intuitive.
I agree that a well done 915 is a great robust trans.
It is also a pleasure to work on. Elegantly engineered and intuitive.
08-22-2005, 02:52 PM
#7
Rennlist Member
Thread Starter
Signed up for new 915 gearbox
I bit the bullet: I ordered a new 915 gearbox from Porsche - actually a factory recon which I will trade against my current gearbox. It does not come cheap but still cheaper than replacing all the worn parts separately in my current gearbox and I hope it will really feel like new. Well like a new 915 anyway which is certainly not the same as the gearbox of a new 997.
I was surprised to be told that there are actually 2 gearbox models for the 85-86 911s with a price difference of ca. US2,000! Mine fortunately seems to be the less costly type. I am not sure what the differences are. I was told that mine has limited slip differential... Anyone volunteers to explain, I would appreciate.
Cheerios
I was surprised to be told that there are actually 2 gearbox models for the 85-86 911s with a price difference of ca. US2,000! Mine fortunately seems to be the less costly type. I am not sure what the differences are. I was told that mine has limited slip differential... Anyone volunteers to explain, I would appreciate.
Cheerios